There is a need for an environmentally biodegradable packaging thermoplastic as an answer to the tremendous amounts of discarded plastic packaging materials. U.S. plastic sales in 1987 were 53.7 billion lbs of which 12.7 billion lbs were listed as plastics in packaging. A significant amount of this plastic is discarded and becomes a plastic pollutant that is a blot on the landscape and a threat to marine life. Mortality estimates range as high as 1-2 million seabirds and 100,000 marine mammals per year.
A further problem with the disposal of plastic packaging is the concern for dwindling landfill space. It has been estimated that most major cities will have used up available landfills for solid waste disposal by the early 1990's. Plastics comprise approximately 3 percent by weight and 6 percent of the volume of solid waste.
One other disadvantage of conventional plastics is that they are ultimately derived from petroleum, which leaves plastics dependent on the uncertainties of foreign crude oil imports. A better feedstock would be one that derives from renewable, domestic resources.
However, there are good reasons for the use of packaging plastics. They provide appealing aesthetic qualities in the form of attractive packages which can be quickly fabricated and filled with specified units of products. The packages maintain cleanliness, storage stability, and other desirable qualities such as transparency for inspection of contents. These packages are known for their low cost of production and chemical stability. This stability, however leads to very long-life of the plastic, so that when its one-time use is completed, discarded packages remain on, and in, the environment for incalculably long times.
It will be appreciated by those skilled in the art that duplicating the properties of one thermoplastic with another is not predictable. Thus, with crystal polystyrene, there are exacting requirements for satisfactory performance of the polystyrene, which has been developed over many years to meet manufacturing and end-use specifications of crystal polystyrene grades.
There are many citations in the prior art for the preparation of lactic acid polymers and copolymers. The earliest processes used lactic acid directly as the monomer, cf., e.g., U.S. Pat. Nos. 1,995,970; 2,362,511; and 2,683,136. The poly(lactic acids) of these patents were of low molecular weights, tacky and without good physical properties. U.S. Pat. No. 2,668,162 (Lowe, DuPont) discloses the use of lactide as the monomer. Lactide is the dilactone of lactic acid and is an internal ester of lactic acid. When lactide is formed, byproduct water is eliminated, permitting the lactide subsequently to be ring-opened polymerized to linear polyester of high molecular weight without tedious condensation methods. Polymers and copolymers of excellent physical properties were obtained by using the intermediate, lactide, to form poly(lactic acid). Copolymers of lactide and glycolide are disclosed by the Lowe patent which are tough, clear, cold-drawable, stretchable, and capable of forming at 210 C. into self-supporting films.
U.S. Pat. No. 2,703,316 discloses lactide polymers which can be a "wrapping tissue" material that is intrinsically stiff and brittle. The lactide monomer is specified as having a melting point greater than 120 C. L-lactide monomer melts at 95 C. and D,L-lactide melts at 128 C.
U.S. Pat. No. 2,758,987 discloses homopolymers of either L- or D,L-lactide which are described as melt-pressable into clear, strong, orientable films. The properties of the poly(L-lactide) are given as: tensile strength, 29,000 psi; percent elongation, 23 percent, tensile modulus 710,000 psi. The poly(D,L-lactide) properties were: 26,000 psi tensile strength; 48 percent elongation; and a tensile modulus of 260,000 psi. Copolymers of L- and D,L-lactide, that is copolymers of L- and D,L-lactic acid, are disclosed only for a 50/50 by weight mixture. Only tack point properties are given (Example 3). It was claimed that one antipodal (optically active, e.g., L-lactide) monomer species is preferred for the development of high strength.
U.S. Pat. No. 2,9510828 discloses a bead polymerization of alpha-hydroxy carboxylic acids such as lactic acid. Copolymers of L- and D,L-lactic are cited at ratios of 75/25, 50/50 and 25/75, respectively. However, no physical properties are given except for particle sizes of the beads and softening points which are all generally in the 110-135 C. range.
U.S. Pat. Nos. 3,297,033; 3,463,158; 3,531,561; 3,636,956; 3,736,646; 3,739,773; and 3,797,499 all disclose lactide polymers and copolymers that are strong crystalline, orientable polymers suitable for fibers and suture materials. These disclosures teach the use of highly-crystalline materials, which are oriented by drawing and annealing to obtain tensile strengths and moduli, typically, greater than 50,000 psi and 1,000,000 psi, respectively. Although formability is mentioned into a variety of shaped articles, physical properties of unoriented extrudates and moldings are not mentioned. For example, U.S. Pat. No. 3,636,956 teaches the preparation of a copolymer having 85/15, 90/10, 92.5/7.5, or a 95/5 weight ratio of L-lactide/D,L-lactide; drawn, oriented fibers are cited; other plasticizers such as glyceryl triacetate, and dibutyl pthalate are tought; however, it is preferred in this disclosure to use pure L-lactide monomer for greater crystallinity and drawn fiber strength; and finally, the advantages of the present invention (e.g. an intimate dispersion of lactic acid based plasticizers that provides unique physical properties) are not obtained.
U.S. Pat. No. 3,797,499 teaches the copolymerization of 95/5 weight ratio, of L-lactide/D,L-lactide (Example V); however, the material is formed into filaments. In column 5, line 1 Schneider teaches against enhanced properties in the range provided in the present invention. Plasticizers such as glyceryl triacetate, ethyl benzoate and diethyl phthalate are used.
Okuzumi et al, U.S. Pat. No. 4,137,921, in Example 4, teaches a 90/10 random copolymer of L-lactide and D,L-lactide, however, the advantages of the present invention are not obtained. Hutchinson, U.S. Pat. No. 4,789,726, teaches a process for the manufacture of polyesters, particularly polylactides of low molecular weight, by forming high molecular weight material and then degrading it to lower weight products of controlled polydispersity, however, monomers are removed in the process.
U.S. Pat. Nos. 3,736,646; 3,773,919; 3,887,699; 4,273,920; 4,471,077; and 4,578,384 teach the use of lactide polymers and copolymers as sustained-drug release matrices that are biodegradable and biocompatible. Again, physical properties of the polymers from ordinary thermoforming methods such as film extrusion or molding are not mentioned.
Other patent art which teach the preparation of L-lactide/D,L-lactide copolymers are Offenlegungsschrift 2,118,127 cites a snow-white, obviously crystalline polymer, no other physical properties were given for this copolymer; Canadian Patent 808,731, Canadian Patent 863,673, and Canadian Patent 923,245. The manufacture of films and fibers from the lactide copolymers is mentioned, but physical property data are limited again to drawn fibers.
Additional related art includes: Low molecular weight poly(D,L-lactide) has been recently added to high molecular weight D,L-lactide along with a drug such as caffeine, salicylic acid, or quinidine, see R. Bodmeier et al, International J. of Pharm. 51, pp. 1-8, (1989). Chabot et al in polymerizing L-lactide and racemic D,L-lactide for medical applications removed residual monomer and lower oligomers, see Polymer, Vol. 24, pp. 53-59, (1983). A. S. Chawla and Chang produced four different molecular weight D,L-lactide polymers but removed monomer for in vivo degradation studies, see Biomat., Med. Dev. Art. Org., 13(3&4), pp. 153-162, (1985-86). Kleine and Kleine produce several low residual monomer, poly(lactic acids) from D,L-lactide while determining lactide levels during the polymerization, see Macromolekulare Chemie, Vol. 30, pp. 23-38, (1959); Kohn et al also makes a low residual monomer product while monitoring the monomer content over time, see Journ. Appl. Polymer Science, Vol. 29, pp. 4265-4277, (1984). M. Vert et al teaches high molecular weight polymers with elimination of residual monomer, see Makromol. Chem., Suppl. 5, pp. 30-41, (1981). M. Vert, in Macromol. Chem., Macromol. Symp. 6, pp.109-122, (1986), discloses similar poly(L-/D,L-lactide) materials, see Table 6, p. 118. In European patent application EP 311,065 (1989) poly(D,L-lactide) is prepared as an implant material for drug delivery during degradation, the material contains drugs, low molecular weight polylactide, and other additives; EP 314,245 (1989) teaches a polylactide having a low amount of residual monomer, the polymer is prepared by polymerization of meso D,L-lactide as a homopolymer or with other lactide monomers, the advantages of the present invention are not obtained; and West German Offenlegungsschrift DE 3,820,299 (1988) teaches the polymerization of meso D,L-lactide with lactides, however, the advantages of the present invention are not obtained.
Of particular interest, U.S. Pat. No. 4,719,246 teaches the blending of homopolymers of L-lactide, D-lactide, polymers or mixtures thereof; and copolymers of L-lactide or D-lactide with at least one nonlactide comonomer. The blending is intended to produce compositions having interacting segments of poly(L-lactide) and poly(D-lactide).